The present embodiments relate to a method for determining an optimal structure of a high-frequency shield of a high-frequency antenna of a magnetic resonance device. The high-frequency shield shields the high-frequency antenna from a gradient-coil arrangement.
High-frequency shields are customarily employed in magnetic resonance devices for shielding the high-frequency antenna, for example a body antenna, from external influences, in particular from an adjacent gradient-coil arrangement. A number of factors may be taken into account in connection with shield structures for high-frequency antennas in magnetic resonance devices.
Good conductivity may be retained for the entire high-frequency-antenna arrangement in the interest of not affecting antenna efficiency. A second desirable factor is a high degree of high-frequency shield tightness for preventing interferences from being coupled into the high-frequency antenna and at the same time for preventing the high-frequency antenna from being coupled with high losses to components behind the shield, such as the gradient coil or PET elements. A third factor is for gradient eddy currents to be adequately suppressed to prevent the shield from overheating and to prevent imaging from being affected, since eddy currents may give rise to artifacts in certain magnetic resonance sequences.
The first two factors may be addressed simultaneously via a closed, conductive area, for example very thin copper foils, although that does not satisfy the third factor.
To address all of the factors, it has been proposed using thin copper foils having a slit structure for suppressing the eddy currents. The use of at least two layers of copper foil may simultaneously be provided, with further layers likewise having slits and being conductive for achieving adequate shield attenuation and a high level of antenna efficiency. The copper layers are separated by a thin dielectric, for example a layer 0.1 mm thick.
The slit design selection and the choice of dielectric are pertinent to addressing the above-cited factors. With currently known development methods or, as the case may be, in the production of high-frequency antennas, the slit design and dielectric are selected empirically, for example based on experience or complex and expensive trials.
DE 10 2007 014 135 A1 discloses a magnetic resonance system having a high-frequency shield that has a frequency-dependent shield effect. The high-frequency shield has a multiplicity of cells, with the cells forming a structure that is repeated two-dimensionally. Each cell is coupled via an impedance to each bordering cell, with the possibility of each impedance being embodied via discrete components. The result is a highly complex structure, which is only determined empirically, and which exhibits completely different characteristics when there are even slight deviations.